In recent years, since the miniaturization of electron devices, such as a transistor, progresses and this size has nanometer size, the new phenomenon called a quantum effect is observed. Then, the development, which aimed at achieving of an ultra high-speed device or a new functional device, is advanced using the quantum effect.
On the other hand, the trial which performs large capacity communication, the information processing, imaging or measurement, etc. is performed in such environment using the frequency domain which is called a terahertz band in particular, and whose frequency is 1 THz (1012 Hz) to 10 THz. This frequency domain is undeveloped frequency region between light and electromagnetic wave, and if the device, which operates with this frequency band, is achieved, being used for many uses, such as measurement in various fields, such as physical characteristics, astronomy, living things, etc. the imaging, the large capacity communication and the information processing mentioned above, is expected.
As a device, which oscillates the high frequency electromagnetic wave of the frequency of the terahertz band, the structure which integrates a minute slot antenna on active devices, such as a transistor and a diode, is known (for example, refer to Non Patent Literature 1).
Moreover, a device, having a structure which adds metal and an insulation layer on a slot transmission line of both terminals of antenna to be short-circuited in high frequency, is disclosed (for example, refer to Non Patent Literatures 1 and 2). The oscillation device has the characteristics, such that the production is easy and it is suitable for the miniaturization.
Moreover, in order to prevent the leakage of a high frequency electromagnetic wave, the method of short-circuiting completely in high frequency and reflecting the leakage by providing a resonance stub in the slot transmission line of antenna both terminals, is proposed. The method by the resonance stub is a method of using that the electromagnetic wave which entered into the transmission line are short-circuited equivalently in the position of the stub by adding the resonant circuit composed of a stub transmission line of the length of a quarter-wavelength to a part of transmission line. According to the method, since it is decided that the length of the stub is a quarter-wavelength against the wavelength of the inputted electromagnetic wave, it is effective in indicating strong resonance only for specific frequency, and short-circuiting only the frequency and reflecting strongly.
FIG. 1 is a schematic bird's-eye view of an oscillation device produced by combining RTD (Resonant Tunneling Diode) and a slot antenna. An active element 109 composed of RTD is placed near the center of a slot antenna 100, metal and an insulator are laminated by the both terminals of the slot antenna 100, and thereby a MIM (Metal Insulator Metal) structure which inserts the insulator with the electrode metal above and below is formed. In this case, the MIM structure is composed of a second electrode 104/an insulating layer 103/a first electrode 102, and is short-circuited in high frequency.
In the second electrode 104, two recessed regions 105 and 106 are formed in the central part overlapping via the first electrode 102 and the insulating layer 103. A protruded region 107 is formed in the state of being inserted into these two recessed regions 105 and 106. A salient region 108 is formed at the approximately central part of the protruded region 107 of the second electrode 104. The active element 109 is placed so as to be inserted between the second electrode 104 and the first electrode 102 at the lower part of the salient region 108. In addition, DC power supply 115 is connected between the second electrode 104 and the first electrode 102, and a parasitic oscillation suppression resistance 114 formed with materials, such as bismuth (Bi), is connected between the second electrode 104 and the first electrode 102 in order to prevent a parasitic oscillation.
SI (Semi-Insulating) InP is used as a component of the semiconductor substrate 101. The slot antenna 100 made at the both sides of the RTD serves as a resonator and a radiation antenna of electromagnetic wave. This oscillation device has a structure where the electromagnetic wave is emitted to an upward and downward two-way for the semiconductor substrate 101. Accordingly, for example, as shown in FIG. 2, it is necessary to newly provide a silicon hemispherical lens 120 for concentrating the electromagnetic wave (hν) emitted to an upward and downward vertical direction.
In the conventional terahertz oscillation device shown in FIG. 1, the active elements 109 placed on the same plane as the insulating layer 103, such as a transistor and a diode, are placed in the central part of the slot antenna 100, the both terminals of a slot transmission line is curved rectangular, and this part is covered by a layered structure of metal/insulator/metal. Accordingly, the part covered by the layered structure of metal/insulator/metal composes RF reflection units 150a and 150b, is short-circuited in high frequency, and thereby the slot antenna 100 is composed. Since this slot antenna 100 is an opened state in direct current, it can supply a direct current to the active element 109.
In FIG. 1 and FIG. 2, since the upper side of the slot antenna 100 is the air, the relative dielectric constant ∈air=1 is satisfied. Since the lower part of the slot antenna 100 uses an InP substrate as the semiconductor substrate 101, the relative dielectric constant ∈InP=12.1 is satisfied. Here, the rate of an oscillation output to the lower part of the slot antenna 100 for whole oscillation output is expressed by ∈InP3/2/(∈air3/2+∈InP3/2)=0.97. That is, about 97% of the oscillation output is emitted to the InP substrate side among the whole oscillation output.
The oscillation of 1.02 THz (1012 Hz) of terahertz bands is achieved at room temperature by the oscillation device of this structure (refer to Non Patent Literature 3). That is, according to the prototyped device, the oscillation frequency of the fundamental wave is 342 GHz, and the output is 23 μW. Then, it is reported that the electromagnetic wave of 1.02 THz is simultaneously outputted as a third higher harmonic wave of a fundamental wave, and the output of this third higher harmonic wave is 0.59 μW (refer to Non Patent Literature 3).
However, since the oscillation device proposed by these Non Patent Literatures 1, 2, and 3 had an insufficient high frequency short circuit structure, there was a problem that the loss of leakage in the direction perpendicular to the direction, where a high frequency electromagnetic wave oscillates (that is, horizontal direction), occurs, and the high output power can not be obtained. That is, in the terahertz oscillation device of structure as shown in FIG. 1, since the electric power of the electromagnetic wave oscillated to the target direction has only about 3%, practical use is not provided.
Moreover, in the method of providing the resonance stub at the slot transmission line of the both terminals of the antenna in order to reflect the leakage of high frequency power, when the active element used for the oscillation device had the characteristic of a frequency variable, and when the electromagnetic wave with comparatively wide bandwidth is oscillated, a high-output power oscillation was not able to be performed by reflecting the leakage power over the whole region of the oscillating frequency. That is, according to the method of providing the stub transmission line of a quarter-wavelength on the slot path which the leakage of electromagnetic wave occurs, even if it is effective in the short circuit and reflection for specific frequency, there was a problem that it could not be effectively reflected for the electromagnetic wave having a frequency band of broadband.
This is because it does not become perfect resonance when the stub transmission line added for the wavelength of the electromagnetic wave oscillated is not in agreement with the length of the quarter wavelength of electromagnetic wave, so the short circuit and the reflection are unsatisfactory. However, even if the length of the stub transmission line is not the quarter wavelength of electromagnetic wave, it is confirmed that a certain amount of weak reflection occurs. Therefore, if such a stub transmission line is added to multi stages at the interval fixed in the middle of the transmission line, the reflection by the added stub transmission line is synthesized. Alternatively, if the phase is uniformed to some extent, the reflection by the added stub transmission line can be reinforced. As a result, the strong reflection can be achieved also for the frequency having wide width not only for specific frequency.
On the other hand, it is disclosed also about a terahertz oscillation device which can eliminate the leakage from a slot transmission line over the oscillation frequency whole region, and can oscillate a high-efficiency and high-output electromagnetic wave even if it is the electromagnetic wave of a frequency band with the comparatively wide bandwidth oscillated from the oscillation device of a frequency variable (for example, refer to Patent Literature 1). In Patent Literature 1, by providing a stub of multi stages in the both ends of the minute slot antenna including an active device composed of RTD etc., reflecting the frequency band of the electromagnetic wave having comparatively wide bandwidth from the multi stage stub, and providing the multi stage stub circuit, since the leakage of the electromagnetic wave oscillated from the active element is reflected and returns to the active element, a high-output oscillation output with wide bandwidth is obtained, and also when the active element used for the oscillation device is a frequency variable, a high oscillation output is obtained corresponding to the frequency variable.
Citation List
    Patent Literature 1: Japanese Patent Application Laying-Open Publication No. 2007-124250    Non-Patent Literature 1: Orihashi, S. Hattori, and M. Asada: “Millimeter and Submillimeter Oscillators Using Resonant Tunneling Diode and Slot Antenna with Stacked-Layer Slot Antenna”, Jpn.J.Appl.Phys. vol. 67, L1309(2004).    Non-Patent Literature 2: Orihashi, S. Hattori, and M. Asada: “Millimeter and Submillimeter Oscillator Using Resonant Tunneling Diode and Slot Antenna with a novel RF short structure”, Int. Conf. Infrared and Millimeter Waves (IRMMW2004), Karlsruhe (Germany), M5.3 (September 2004) pp. 121-122.    Non-Patent Literature 3: Orihashi, S. Suzuki, and M. Asada: “Harmonic Generation of 1 THz in Sub-THz Oscillating Resonant Tunneling Diode”, IRMMW2005/THz2005, The Joint 30th International Conference on Infrared and Millimeter Waves and 13th Infrared Conference on Terahertz Electronics, 2005.9.19-23